Troubleshooting the Docker daemon

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Troubleshooting
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This page describes how to troubleshoot and debug the daemon if you run into issues.

You can turn on debugging on the daemon to learn about the runtime activity of the daemon and to aid in troubleshooting. If the daemon is unresponsive, you can also force a full stack trace of all threads to be added to the daemon log by sending the SIGUSR signal to the Docker daemon.

Daemon

Unable to connect to the Docker daemon

Cannot connect to the Docker daemon. Is 'docker daemon' running on this host?

This error may indicate:

  • The Docker daemon isn't running on your system. Start the daemon and try running the command again.
  • Your Docker client is attempting to connect to a Docker daemon on a different host, and that host is unreachable.

Check whether Docker is running

The operating-system independent way to check whether Docker is running is to ask Docker, using the docker info command.

You can also use operating system utilities, such as sudo systemctl is-active docker or sudo status docker or sudo service docker status, or checking the service status using Windows utilities.

Finally, you can check in the process list for the dockerd process, using commands like ps or top.

Check which host your client is connecting to

To see which host your client is connecting to, check the value of the DOCKER_HOST variable in your environment.

$ env | grep DOCKER_HOST

If this command returns a value, the Docker client is set to connect to a Docker daemon running on that host. If it's unset, the Docker client is set to connect to the Docker daemon running on the local host. If it's set in error, use the following command to unset it:

$ unset DOCKER_HOST

You may need to edit your environment in files such as ~/.bashrc or ~/.profile to prevent the DOCKER_HOST variable from being set erroneously.

If DOCKER_HOST is set as intended, verify that the Docker daemon is running on the remote host and that a firewall or network outage isn't preventing you from connecting.

Troubleshoot conflicts between the daemon.json and startup scripts

If you use a daemon.json file and also pass options to the dockerd command manually or using start-up scripts, and these options conflict, Docker fails to start with an error such as:

unable to configure the Docker daemon with file /etc/docker/daemon.json:
the following directives are specified both as a flag and in the configuration
file: hosts: (from flag: [unix:///var/run/docker.sock], from file: [tcp://127.0.0.1:2376])

If you see an error similar to this one and you are starting the daemon manually with flags, you may need to adjust your flags or the daemon.json to remove the conflict.

Note

If you see this specific error message about hosts, continue to the next section for a workaround.

If you are starting Docker using your operating system's init scripts, you may need to override the defaults in these scripts in ways that are specific to the operating system.

Configure the daemon host with systemd

One notable example of a configuration conflict that's difficult to troubleshoot is when you want to specify a different daemon address from the default. Docker listens on a socket by default. On Debian and Ubuntu systems using systemd, this means that a host flag -H is always used when starting dockerd. If you specify a hosts entry in the daemon.json, this causes a configuration conflict and results in the Docker daemon failing to start.

To work around this problem, create a new file /etc/systemd/system/docker.service.d/docker.conf with the following contents, to remove the -H argument that's used when starting the daemon by default.

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd

There are other times when you might need to configure systemd with Docker, such as configuring a HTTP or HTTPS proxy.

Note

If you override this option without specifying a hosts entry in the daemon.json or a -H flag when starting Docker manually, Docker fails to start.

Run sudo systemctl daemon-reload before attempting to start Docker. If Docker starts successfully, it's now listening on the IP address specified in the hosts key of the daemon.json instead of a socket.

Important

Setting hosts in the daemon.json isn't supported on Docker Desktop for Windows or Docker Desktop for Mac.

Out of memory issues

If your containers attempt to use more memory than the system has available, you may experience an Out of Memory (OOM) exception, and a container, or the Docker daemon, might be stopped by the kernel OOM killer. To prevent this from happening, ensure that your application runs on hosts with adequate memory and see Understand the risks of running out of memory.

Kernel compatibility

Docker can't run correctly if your kernel is older than version 3.10, or if it's missing kernel modules. To check kernel compatibility, you can download and run the check-config.sh script.

$ curl https://raw.githubusercontent.com/docker/docker/master/contrib/check-config.sh > check-config.sh

$ bash ./check-config.sh

The script only works on Linux.

Kernel cgroup swap limit capabilities

On Ubuntu or Debian hosts, you may see messages similar to the following when working with an image.

WARNING: Your kernel does not support swap limit capabilities. Limitation discarded.

If you don't need these capabilities, you can ignore the warning.

You can turn on these capabilities on Ubuntu or Debian by following these instructions. Memory and swap accounting incur an overhead of about 1% of the total available memory and a 10% overall performance degradation, even when Docker isn't running.

  1. Log into the Ubuntu or Debian host as a user with sudo privileges.

  2. Edit the /etc/default/grub file. Add or edit the GRUB_CMDLINE_LINUX line to add the following two key-value pairs:

    GRUB_CMDLINE_LINUX="cgroup_enable=memory swapaccount=1"

    Save and close the file.

  3. Update the GRUB boot loader.

    $ sudo update-grub

    An error occurs if your GRUB configuration file has incorrect syntax. In this case, repeat steps 2 and 3.

    The changes take effect when you reboot the system.

Networking

IP forwarding problems

If you manually configure your network using systemd-network with systemd version 219 or later, Docker containers may not be able to access your network. Beginning with systemd version 220, the forwarding setting for a given network (net.ipv4.conf.<interface>.forwarding) defaults to off. This setting prevents IP forwarding. It also conflicts with Docker's behavior of enabling the net.ipv4.conf.all.forwarding setting within containers.

To work around this on RHEL, CentOS, or Fedora, edit the <interface>.network file in /usr/lib/systemd/network/ on your Docker host, for example, /usr/lib/systemd/network/80-container-host0.network.

Add the following block within the [Network] section.

[Network]
...
IPForward=kernel
# OR
IPForward=true

This configuration allows IP forwarding from the container as expected.

DNS resolver issues

DNS resolver found in resolv.conf and containers can't use it

Linux desktop environments often have a network manager program running, that uses dnsmasq to cache DNS requests by adding them to /etc/resolv.conf. The dnsmasq instance runs on a loopback address such as 127.0.0.1 or 127.0.1.1. It speeds up DNS look-ups and provides DHCP services. Such a configuration doesn't work within a Docker container. The Docker container uses its own network namespace, and resolves loopback addresses such as 127.0.0.1 to itself, and it's unlikely to be running a DNS server on its own loopback address.

If Docker detects that no DNS server referenced in /etc/resolv.conf is a fully functional DNS server, the following warning occurs:

WARNING: Local (127.0.0.1) DNS resolver found in resolv.conf and containers
can't use it. Using default external servers : [8.8.8.8 8.8.4.4]

If you see this warning, first check to see if you use dnsmasq:

$ ps aux | grep dnsmasq

If your container needs to resolve hosts which are internal to your network, the public nameservers aren't adequate. You have two choices:

  • Specify DNS servers for Docker to use.

  • Turn off dnsmasq.

    Turning off dnsmasq adds the IP addresses of actual DNS nameservers to /etc/resolv.conf, and you lose the benefits of dnsmasq.

You only need to use one of these methods.

Specify DNS servers for Docker

The default location of the configuration file is /etc/docker/daemon.json. You can change the location of the configuration file using the --config-file daemon flag. The following instruction assumes that the location of the configuration file is /etc/docker/daemon.json.

  1. Create or edit the Docker daemon configuration file, which defaults to /etc/docker/daemon.json file, which controls the Docker daemon configuration.

    $ sudo nano /etc/docker/daemon.json

  2. Add a dns key with one or more DNS server IP addresses as values.

    {
  "dns": ["8.8.8.8", "8.8.4.4"]
}

    If the file has existing contents, you only need to add or edit the dns line. If your internal DNS server can't resolve public IP addresses, include at least one DNS server that can. Doing so allows you to connect to Docker Hub, and your containers to resolve internet domain names.

    Save and close the file.

  3. Restart the Docker daemon.

    $ sudo service docker restart

  4. Verify that Docker can resolve external IP addresses by trying to pull an image:

    $ docker pull hello-world

  5. If necessary, verify that Docker containers can resolve an internal hostname by pinging it.

    $ docker run --rm -it alpine ping -c4 <my_internal_host>

PING google.com (192.168.1.2): 56 data bytes
64 bytes from 192.168.1.2: seq=0 ttl=41 time=7.597 ms
64 bytes from 192.168.1.2: seq=1 ttl=41 time=7.635 ms
64 bytes from 192.168.1.2: seq=2 ttl=41 time=7.660 ms
64 bytes from 192.168.1.2: seq=3 ttl=41 time=7.677 ms

Turn off dnsmasq

If you prefer not to change the Docker daemon's configuration to use a specific IP address, follow these instructions to turn off dnsmasq in NetworkManager.

  1. Edit the /etc/NetworkManager/NetworkManager.conf file.

  2. Comment out the dns=dnsmasq line by adding a # character to the beginning of the line.

    # dns=dnsmasq

    Save and close the file.

  3. Restart both NetworkManager and Docker. As an alternative, you can reboot your system.

    $ sudo systemctl restart network-manager
$ sudo systemctl restart docker

To turn off dnsmasq on RHEL, CentOS, or Fedora:

  1. Turn off the dnsmasq service:

    $ sudo systemctl stop dnsmasq
$ sudo systemctl disable dnsmasq

  2. Configure the DNS servers manually using the Red Hat documentation.

Volumes

Unable to remove filesystem

Error: Unable to remove filesystem

Some container-based utilities, such as Google cAdvisor, mount Docker system directories, such as /var/lib/docker/, into a container. For instance, the documentation for cadvisor instructs you to run the cadvisor container as follows:

$ sudo docker run \
  --volume=/:/rootfs:ro \
  --volume=/var/run:/var/run:rw \
  --volume=/sys:/sys:ro \
  --volume=/var/lib/docker/:/var/lib/docker:ro \
  --publish=8080:8080 \
  --detach=true \
  --name=cadvisor \
  google/cadvisor:latest

When you bind-mount /var/lib/docker/, this effectively mounts all resources of all other running containers as filesystems within the container which mounts /var/lib/docker/. When you attempt to remove any of these containers, the removal attempt may fail with an error like the following:

Error: Unable to remove filesystem for
74bef250361c7817bee19349c93139621b272bc8f654ae112dd4eb9652af9515:
remove /var/lib/docker/containers/74bef250361c7817bee19349c93139621b272bc8f654ae112dd4eb9652af9515/shm:
Device or resource busy

The problem occurs if the container which bind-mounts /var/lib/docker/ uses statfs or fstatfs on filesystem handles within /var/lib/docker/ and does not close them.

Typically, we would advise against bind-mounting /var/lib/docker in this way. However, cAdvisor requires this bind-mount for core functionality.

If you are unsure which process is causing the path mentioned in the error to be busy and preventing it from being removed, you can use the lsof command to find its process. For instance, for the error above:

$ sudo lsof /var/lib/docker/containers/74bef250361c7817bee19349c93139621b272bc8f654ae112dd4eb9652af9515/shm

To work around this problem, stop the container which bind-mounts /var/lib/docker and try again to remove the other container.